This invention relates to a mass spectrometer device for measuring isotopomers precisely. An isotopomer is a molecular species comprising an isotope in the molecule. About 10 types of isotopomers exist in greenhouse gas due to combinations of elements and inner atomic positions, and the number increases exponentially in polymers such as those found in organic compounds from living things on the ocean floor and on the land.
A method has been proposed, as disclosed for example in Japanese Patent Hei 3-52180, which not only applies a polarizing magnetic field to target ions, but also applies a toroidal electric field and stigmatic second order double focusing to perform efficient analysis. The resulting ion analysis offers high sensitivity and stable performance for various types of ions.
However, insufficient consideration had been given to the precise and convenient measurement of isotopomers.
The analysis of isotopomers is generally performed as follows.
An unknown sample and a standard are converted to gaseous molecules, and these are introduced into a mass spectrometer where they are ionized by electron impact. In this case, to compare their ion currents, the unknown sample and the standard are introduced to the ion source alternately in short time intervals. A mass analysis is then performed by a magnetic sector-type mass spectrometer having an orbital radius of the order of 5-20 cm. The mass spectrometer employs multiple collectors, the abundance ratios of molecular species including isotopes being detected by the ion currents detected by these collectors.
In the mass analysis of isotopomers, a xcex4 value is usually used to represent the isotope content of the sample. The xcex4 value represents the difference of an isotope ratio relative to a standard by a permillage (%). Taking oxygen as an example, this is given by the following equation (1).                               δ          ⁢                                                    xe2x80x83                                      18                    ⁢          O                =                              {                                                                                (                                                                                                                       18                                                ⁢                        O                                                                                                                         16                                                ⁢                        O                                                              )                                    Sample                                -                                                      (                                                                                                                       18                                                ⁢                        O                                                                                                                         16                                                ⁢                        O                                                              )                                    SMOW                                                                              (                                                                                                             18                                            ⁢                      O                                                                                                               16                                            ⁢                      O                                                        )                                SMOW                                      }                    xc3x97                      10            3                    ⁢                      xe2x80x83                    ⁢                      (            %            )                                              (        1        )            
Here, SMOW is an abbreviation for standard Men Ocean Water, and is used worldwide as a standard sample for oxygen and hydrogen.
The ion current introduced into the multiple collectors is measured by the direct method. For example, in the case of CO2 gas, ions having an m/e (mass/charge)=44 are CO2+, and as they are much more abundant than ions of other m/e values, an ion current I1 (m/e=44) incident on the first collector is stronger by an order of magnitude than an ion current I2 (m/e=45) incident on the second collector. These are read directly for both the standard gas and the sample gas, and the xcex4 value is calculated from their ratio.                               δ          m                =                                                                              I                  2                                                  I                  1                                            -                                                (                                                            I                      2                                                              I                      1                                                        )                                wst                                                                    (                                                      I                    2                                                        I                    1                                                  )                            wst                                xc3x97                      10            3                    ⁢                      xe2x80x83                    ⁢                      (            %            )                                              (        2        )            
Here, the suffix WST refers to a standard used in the laboratory.
In the magnetic type single focusing mass spectrometer have multiple collectors which was previously used for the mass analysis of isotopomers, the mass resolution was extremely low, being only of the order of 100 to 200. In a mass spectrometer having only this degree of mass resolution, in the case of dinitric oxide (N2O) for example, it is impossible to separately detect 14N15N16O (molecular weight 44.99809760) and 14N217O (molecular weight 45.0052790). In other words, the mass spectrometry of isotopomers could not be performed.
As an example of the mass resolution required for the mass spectrometry of isotopomers, Table 1 shows results calculated from data in the scientific annals of the National Astronomical Observatory of Japan in the case of methane, dinitric oxide and nitric oxide.
This table shows combinations of component elements and molecular weights for these molecules. As seen from the table, there is very little difference in molecular weights, and it is easily appreciated that a high mass resolution is required to detect them separately.
In the above Table 1, only molecular weights are shown, but another problem is that the abundances of these ions are very different. As an example, Table 2 shows the abundance ratios of isotopomers for the molecule N2O. This data was calculated from the data in the aforesaid scientific annals.
If the single focusing mass spectrometer having multiple collectors of the prior art were to have a high mass resolution, for example 10,000 or higher, it would be a very large device wherein the distance between the ion source of the mass spectrometer and the detector was of the order of several tens of meters. Further, as it would not be able to deal with extreme differences of abundance ratios, it would not be practically feasible.
To perform the mass analysis of isotopomers, this invention is based on the double focusing mass spectrometer disclosed in Japanese Patent Hei 3-52180. For simple analysis of molecules comprising stable isotopes of the same element, part of the ion accelerating voltage is scanned. For the analysis of isotopomers with different elements, the magnetic field intensity is changed to a value corresponding to the particular element before part of the ion accelerating voltage is scanned. For extreme differences of abundance ratios, an amplifier is also used for signal detection wherein the gain is varied according to the abundance ratio.